1. Field of the Invention
The present invention relates to an LED (light-emitting diode) driver and, more specifically, to an LED driver for high current mode applications.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices including personal computers and high definition TVs, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
A common application for LEDs is in backlighting Liquid Crystal Displays (LCDs). In these applications, a large number of LED strings are typically used (e.g., 64 LED channels), with each string driven by an LED driver. In a conventional LED driver architecture, a fixed resistor in the LED power path limits peak current through the LED string. A PWM-controlled transistor in the LED power path controls the on and off times of the LEDS to achieve a desired brightness.
A problem with the conventional LED driver architecture is that the overall power consumption can be undesirably high. Generally, the power consumption, PWRn, in an LED string, n, is given by:PWRn=VLEDn×ILEDn  (1)where VLEDn is the voltage across LED string n and ILEDn is the current through LED string n. The total power consumption PWRtotal of a circuit having N LED channels is given by:
                              PWR          total                =                              ∑                          n              =              1                        N                    ⁢                                          ⁢                      PWR            n                                              (        2        )            
The LED voltage VLEDn (and therefore the power consumption PWRn) in an LED string is a function of the resistance of the fixed resistor in the LED power path and the turn-on impedance of the PWM transistor. Because the turn-on impedance of the PWM transistor increases as LED current increases, the overall power consumption becomes particularly high in high current mode applications such as 3D mode or scan mode operation for LCDs, where the LED current ILEDn can rise up to, for example, 450 mA with a 20% duty cycle. In conventional LED driver architectures, operating at these current levels can create thermal problems in the LED driver chip due to the high power consumption. The problem can be further exacerbated by poor ground noise rejection associated with some conventional LED driver architectures, thereby limiting overall performance and robustness.